The present invention relates to an ion deposition web-fed print engine having a novel and improved erase unit for removing residual electrostatic potential of an image remaining on the engine's image cylinder after the toner developed latent image has been transferred to a substrate.
Ion deposition printers conventionally transpose or transform computer-generated signals, such as word processing signals, for image printing on a substrate, for example, paper. More particularly, an ion deposition print engine typically includes an image cylinder mounted in opposition to an impression cylinder, with the substrate, i.e., a web of paper, passing between the image and impression cylinders. The image cylinder includes a dielectric layer which receives an electrostatic image from an ion cartridge. The cartridge is driven electronically from the computer or word processing system. The electrostatic image imposed on the image cylinder is contacted with toner from a supply. At the nip between the image and impression cylinders, the toner is transferred to the substrate, i.e., the paper, in the identical form of the electrostatic image on the image cylinder and fused to the substrate. Further rotation of the image cylinder causes it to pass a multi-component cleaning station, which physically removes residual solid particulate matter (i.e., toner). The image cylinder finally passes in opposition to an erase unit, which removes any residual electrostatic potential of the image on the image cylinder surface, whereby a fresh electrostatic image may be placed on the dielectric layer by the ion cartridge. The process is then repeated with the same or different images. The present invention is particularly concerned with a novel and improved erase unit for the ion deposition print engine.
Presently known erase units for ion deposition print engines use a high-density ion current generator to erase the latent residual electrostatic image remaining on the image cylinder after transfer of the image to the substrate. One such known erase unit comprises a central glass rod with four individual glass-coated erase wires mounted 90.degree. apart around the central glass rod and wrapped with a spiral-coiled screen wire. To erase the residual electrostatic potential remaining on the image cylinder, the erase wire is activated by application of high voltage RF energy. This causes atmospheric breakdown and ionization on the surface of the glass-coated erase wire at the junctions of the spiral screen wire. The resultant pool of ions, both positive and negative, migrate to the residual electrostatic image areas on the drum surface as a result of the net electrical field present between the screen wire and the residual electrostatic images.
While acceptable in that configuration, the above-described erase unit has certain limitations. For example, the life of the erase unit is somewhat limited. When one of the erase wires is no longer effective, the unit is rotated 90.degree. to bring the adjacent wire into close proximity with the drum. A disadvantage with this type of erase unit is the downtime involved in order to displace the next wire into position. Also, the glass-coated wire with the spiral wire wrapping is prone to contamination and readily and easily damaged. If contaminated, the erase unit is substantially non-recoverable. Further, there is a limitation in the voltage range for pre-charging the image cylinder. Still further, the operation of this known erase unit is in ambient conditions. This makes it prone to unusual and undesirable deposition of ionic compounds, particularly in ammonia and amine-laden atmospheres. Moreover, the operation is at relatively low frequency, thus limiting overall output.
According to the present invention, there is provided a novel and improved erase unit for an ion deposition print engine which minimizes or eliminates the foregoing and other problems associated with prior erase units for similar type print engines. Particularly, the present invention provides front and rear, or first and second, electrodes and a circuit for providing a time-varying potential across the electrodes. The first electrode may form the base of a plenum into which inert gas, preferably argon, is provided for generating positive and negative ions within the plenum adjacent the image surface containing the residual electrostatic potential in response to the creation of an electric field within the plenum. The second, or front electrode, also called the biasing electrode, is disposed within the plenum and separated from the first, or rear, electrode by a dielectric, for example, formed of glass. Side and end walls are also provided to further define the plenum whereby the region within the plenum filled with the argon (inert) gas lies in contact with the image cylinder. When the circuit is activated, positive and negative ions are generated adjacent the second, or bias, wire and the electric field between the bias wire and the image drum surface provides the driving force for those ions of appropriate polarity to migrate to the cylinder. The ions created within the plenum are also under the influence of the electric field created by the second electrode and the image cylinder assembly by a DC biasing voltage. That field is a function of the residual image cylinder voltage and the erase bias on the second electrode and the distance between the second electrode and the image surface. As long as there is a difference between the residual image cylinder voltage and the erase bias on the second electrode, a net ion migration to the image cylinder surface occurs. As the image cylinder voltage reaches the value of the erase bias by the charging or discharging of the net ionic migration, the ion current will stop. Thus, in a pure eraser application, the bias or second electrode wire is held near a ground potential to produce a zero volt condition on the image drum. It is, however, also important in certain applications to adjust a pre-biasing potential to a specific level for use with other parts of the imaging and development process. Thus, the erase bias potential can be set to a specific level necessary for another part of the process and the image cylinder will be charged or discharged to that desired level. That is, by driving the second wire with the DC bias, the residual image potential on the drum is erased and brought to a biased condition with a surface voltage matching that of the bias wire.
By using a system of the foregoing described type, there is provided an improved apparatus demonstrating higher density ionic output based on the use of inert gas, affording higher frequency RF energy and an improved configuration of the bias wire, resulting in an erasing operation at higher print speeds and a more efficient eraser mechanism. Additionally, the image cylinder may be pre-charged to a wide range of DC surface voltages by biasing the bias wire and creating a net electric field between the wire and the cylinder. Further, the erase unit hereof is substantially insensitive to harmful gases in the ambient environment and creates an equal and uniform output along its length due to its simple construction and the use of the inert gas environment. Still further, the improved eraser unit hereof affords greater operational longevity in comparison with the previously described eraser units because of the insensitivity of the materials used to degradation over time and the robust nature of the plasma-generating components, hence achieving less sensitivity to contamination and affording the capability of cleaning the unit should it become contaminated.
In a preferred embodiment according to the present invention, there is provided an electrostatic ion deposition printer including an electrostatic print head for forming an electrostatic image, an image cylinder rotatable about an axis and having a dielectric layer for receiving the electrostatic image and means for transferring the image to a substrate, an erase unit for removing residual electrostatic potential of the image remaining on the image cylinder after the image has been transferred to the substrate, comprising first and second electrodes disposed adjacent a surface of the image cylinder at a location in opposition thereto and to the residual electrostatic potential remaining on the image cylinder, a dielectric disposed between the first and second electrodes and means for introducing a gas in a region adjacent the second electrode and between the dielectric and the image cylinder surface. Circuit means provide a time varying potential across the electrodes to ionize the gas in the region and enable substantial equalization of the residual potential on the image cylinder surface and the potential on the second electrode.
In a further preferred embodiment according to the present invention, there is provided an electrostatic ion deposition printer including an electrostatic print head for forming an electrostatic image, an image cylinder rotatable about an axis and having a dielectric layer for receiving the electrostatic image, means for transferring the image to a substrate and an erase unit, including first and second electrodes disposed adjacent a surface of the image cylinder at a location in opposition thereto and to the residual electrostatic potential remaining on the image cylinder and a dielectric disposed between the first and second electrodes, a method for removing residual electrostatic potential remaining on the image cylinder after the image has been transferred to the substrate, comprising the steps of introducing a gas in a region adjacent the second electrode and between the dielectric and the image cylinder surface and providing a time varying potential across the electrodes to ionize the gas in the region and enable substantial equalization of the residual potential on the image cylinder surface and the potential on the second electrode.
Accordingly, it is a primary object of the present invention to provide a novel and improved erase unit for an ion deposition web-fed print engine.
These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings.